Light has several basic properties such as brightness, wavelength (or color), and polarization. In optical devices we would like to control all of them and by modulation of these properties to signal something meaningful to a recipient on another end of a waveguide.
Polarization maintenance (or mode selectivity) is an important feature in light transmitting devices because of requirements imposed by coherent detection systems like homodynes and heterodynes (J. M. Senior, Optics Fiber Communications, N.Y., 1992, 908p).
In other terms, we can say that the combination of optical parameters in non-uniform planar waveguides or fibers imposes restrictions on the existence or energy-occupation level of certain modes, and creates useful properties of the guide or fiber. Industry has gone a long way at substantial expense in its efforts to create selective fibers with a certain configuration of birefringence in the fiber core. These fibers are named Polarization Maintaining Fibers (Ivan. P. Kaminow, “Polarization in Optical Fibers”, IEEE Journal of Quantum Electronics, Vol. QE-17, No. 1, January 1981, 15-221). The fibers maintain certain transmission modes or combinations of modes that have specific spatial and field vector distribution of electromagnetic fields inside a guide or fiber. We will use in this application the terminology that is accepted in papers and patents describing mode structures of light transmission (J. D. Dai, C. K. Jen, Analysis of cladded uniaxial single-crystal fibers. J. Optical Soc. Am. A. 2022-2026; A. Tonning, Circularly symmetrical optical waveguide with strong anisotropy. IEEE Trans. Microwave Theory Technol. MTT-30, 790-794, 1982 John A. Buck, “Fundamentals of Optical Fibers”, John Wiley & Sons, New York, p. 259) is the most appropriate from our perspective for a description of our invention. Another two publications that are highly relevant to this invention are Sorin, et al. U.S. Pat. No. 4,721,352, and R. A. Bergh, H. C. Lefevre, H. J. Shaw, “Single-mode fiber-optic polarizer”, Optics Letters, 1880, vol. 5, No. 11, 479-481.
The definition of light-transmitting devices or waveguides in the present invention is very broad. It encompasses everything that transmits light between a polarized light source and a detector such as a heterodyne or a homodyne. A polarized light source is understood as a laser or un-polarized light source with a polarization controller. An un-polarized light source is any source of light, for example a light-emitting diode, a lamp, sun light, or day light.
In conventional optical systems, polarization is maintained by introducing birefringent elements in the system such as an asymmetrical core in Polarization Maintaining Fibers. However, it creates another problem, namely polarization dispersion of the signal. The present invention provides polarization maintenance in an optical system without the intrinsic problem of polarization dispersion. The present invention inhibits transmission of one of the modes (TE or TM) in optical guides, and provides components that transmit a polarized signal of one polarization while eliminating light of another polarization. These components might be used as polarization controllers, polarizersii and polarization-maintaining transmissive guides such as plane flat guides and fibers.
It is known from literature (R. A. Bergh, et. al. “Single-mode fiber optic polarizers”, Optics Letters, vol. 5, No. 11, 1980, pp. 479-481) that crystalline material can create certain polarization specificity in waveguides such as fiber, for example. The similar specificity is provided by core and cladding asymmetry that is created by the design of the fiber itself as is shown by photonic crystals and “polarization maintaining fibers” (A. Ferrando et al., “Vector description of higher-order modes in photonic crystal fibers”, J. Opt. Soc. Am. A/Vol. 17, No. 7, July 2000, 1333-1340). It is also known (P. Yeh and C. Gu, “Optics of Liquid Crystal Displays”, N.Y., 1999, 427) that some crystals have polarizing capability and maintain polarization of the light.